Aircraft approach and descent procedures are nowadays used to determine the reference path and guide the aircraft between cruising flight and landing. In the context of civil aviation, an approach procedure involves determining a horizontal flight profile, time and altitude constraints associated with the various waypoints of this horizontal path, and determining an associated vertical profile.
The horizontal profile is generally established according to procedures that are specific to each airport, using navigation databases. Thus, the aircraft has to overfly successively navigation points or beacons in a predetermined order in order to reach the runway for landing. In the context of the predefined approach procedures, each of these points is generally associated with one or more constraints, simultaneously relating to time, altitude, gradient or speed. These constraints are extremely important to air traffic control because they make it possible to ensure that approaching aircrafts will descend progressively until they land, while at the same time maintaining enough of a distance or separation from one another that safety is not compromised.
The descent procedures notably use the following altitude constraints: “AT” indicates that the aircraft must overfly a navigation point at a precise altitude; “AT OR ABOVE” indicates that the aircraft must overfly a navigation point at an altitude at least equal to the given altitude; “AT OR BELOW” indicates that the aircraft must overfly a navigation point at an altitude at most equal to the given altitude; “WINDOW” indicates that the aircraft must overfly the navigation point at an altitude that falls within a window arranged between a minimum altitude and a maximum altitude.
Once the navigation points and associated constraints are known, the Flight Management System, commonly known by its acronym FMS, determines a vertical profile that allows each of the navigation points to be validated with the associated time and altitude constraints while at the same time complying with the aircraft flight envelope and decelerating gradually until the aircraft lands.
The method generally adopted in the prior art is a descent referred to as a “stepped descent”. Such a descent consists, as soon as an altitude constraint is reached, in beginning a descent phase to arrive at the next altitude constraint even before the associated navigation point, then flying “level” (at constant altitude) as far as this point, then beginning a new phase of descent followed by level flight, and so on, right up to the final approach. The aircraft therefore uses the level phases to decelerate and “apply configuration” which means to say adapt its aerodynamic configuration, for example by deploying the slats, flaps and landing gears, in order progressively to increase its capacity to decelerate and its lift at low speed.
Stepped descent procedures have the advantage of making it easier to calculate the vertical path, by separating the descent phases from the deceleration phases which are performed during level flight. However, the vertical path produced using these procedures is suboptimal. This is because stepped descent causes the aircraft to fly as low as possible with respect to the altitude constraints to which it is subject. Stepped descents have major disadvantages: firstly, the aircraft fuel consumption is higher at low altitude; secondly the noise generated by the engines and the flow of air around the aircraft (aerodynamic noise) is produced closer to the ground, in zones near the airports which are often densely populated. In addition, in this type of descent, the aircraft decelerates earlier than is necessary and therefore flies at low speed for longer. It is therefore obliged, in order to maintain lift, to switch to a high-lift configuration, namely to fly with the slats and flaps deployed. This type of configuration increases aerodynamic drag and thus necessitates an increase in engine thrust, and therefore in fuel consumption. Thus, stepped descents increase both aircraft fuel consumption and noise pollution associated with the approach of the aircraft.
In order to alleviate these disadvantages, flight procedures known as “CDA” (Continuous Descent Approach), “CDO” (Continuous Descent Operations) or even “OPD” (Optimized Profile Descent) propose the construction of segment flight segments within level segments for the approach procedures. This notably allows the aircraft to fly higher with minimal thrust, while at the same time delaying the application of an aerodynamic configuration, namely deployment of aerodynamic elements that improve low-speed lift but increase aircraft drag and aerodynamic noise at a given speed, thereby decreasing the amount of time spent flying in high-lift configuration. The flight segments of a flight procedure of the type referred to as CDA may notably be performed in “FPA/SPEED” mode, which means to say with an FPA (Flight Path Angle) that is constant. Descent procedures referred to as CDA are notably described in patent U.S. Pat. No. 8,126,599.
However, too simple a construction of optimized flight segments may lead to paths that are unflyable. Specifically, flying as high as possible may lead to the need ultimately to descend at too steep a gradient. This may occur in unforeseen circumstances (for example if a tail wind is a few knots higher than was initially laid down) or quite simply if the angle of descent has been calculated to obtain a profile which is highly optimized for consumption but is not flyable, for example if there is a discrepancy between the actual performance of the aircraft and the model thereof. In such cases, the aircraft may fail to have sufficient capacity to decelerate to be able to meet the constraints of the flight plan without leaving its flight envelope. The pilot may then find himself forced to use the air brakes, rendering the approach suboptimal or may even find himself having to reapply the throttle belatedly, with disastrous consequences in terms of timing of the landing, fuel consumption and noise pollution and therefore cost for the airline.
One solution is to apply safety margins during the construction of the constant ground gradient path, for each element that may have an impact on the descent, for example applying a margin with respect to the possible change in tail wind, a margin with respect to the capacity of the aircraft to decelerate, etc. In order to hold all of these margins, the FMS will need to calculate anticipated moments for applying configuration so as to have a capacity of deceleration sufficient to hold all these margins. This application of configuration increasing the drag of the aircraft, it will therefore be necessary to increase the engine thrust, which will increase both fuel consumption and approach noise.
It is one object of the present invention to obtain a vertical path for approaches of CDA type which is as optimized as possible in terms of fuel consumption and therefore CO2 emissions, flight time and noise pollution, while still remaining flyable with respect to the aerodynamic capacity of the aircraft and a set of performance and safety criteria and the margins associated therewith.